The aims of this project are to understand how substances of bacterial origin initiate the cellular responses of inflammation and enhanced immunity. We prepared and studied the biological activities of several highly purified forms of peptidoglycan, an important cellular constituent of all bacteria and a potent inflammatory stimulus. Biological studies on macrophages from mice, guinea pigs, and humans reveal that both the primary structure of peptidoglycan and the degrees of polymerization and cross-linking determine their biological activity. The most potent initiators are cross-linked polymers; these substances cause higher amounts of the inflammatory mediators, Interleukin 1 (IL-1) and colony-stimulating factors (CSF), to be secreted by macrophages than other forms of peptidoglycan. Indeed, some macrophages only respond to this form. Soluble, noncross-linked polymers are more active at equivalent molar concentrations than monomers. Differences in activity were also found between monomers of different primary structure. These data are consistent with the studies of M. Karnovsky and ourselves, which suggest that macrophages possess two kinds of surface receptors for peptidoglycan, a few of high affinity and more of low affinity. We hypothesize that the monomers and linear polymers only bind sufficiently with the high-affinity receptors, whereas the heavily cross-linked polymers bind to both classes. This enables the cells to respond strongly to infections when they encounter the intact bacteria and only moderately to soluble breakdown products of bacteria. We found that normal human subjects can be divided into two groups, those that produce considerable IL-1 spontaneously and others that require significant stimulation. These data may explain why some human subjects are more likely than others to develop diseases characterized by excessive host responses. (CS)